1. Field of the Invention
The present invention relates to a process for preparing urea-formaldehyde resins containing less than 0.2 ppm free formaldehyde.
2. Prior Art
U.S. Pat. No. 2,729,611 discloses preparing a urea-formaldehyde resin by heating an aqueous mixture of 1.4-1.9 moles of formaldehyde with 1 mole of urea at a pH of at least about 7 to form dimethylol urea, adding formaldehyde to bring the mole ratio of formaldehyde: urea to 2.1:1 to 3.1, reducing the pH to 2.5-6 while continuing the warming, discontinuing the warming and neutralizing the product.
U.S. Pat. No. 3,896,087 discloses the preparation of urea-formaldehyde resins by condensing urea and formaldehyde in aqueous acid solution in a molar ratio of from 1:1.5 to 1:2.5, further condensing at least 0.2 mole of formaldehyde per mole of urea, and adding urea to bring the molar ratio of urea to formaldehyde to from 1:1.5 to 1:2.5 at a pH between 4.0 and 6.5.
U.S. Pat. No. 4,174,310 discloses preparing urea-formaldehyde resins by condensing urea:formaldehyde in a mole ratio of from 1:3 to 1:1.5 in an aqueous solution under neutral or alkaline conditions, vacuum evaporating the reaction mixture, and further condensing the mixture optionally with additional urea under acid conditions.
U.S. Pat. No. 4,246,152 discloses preparing urea-formaldehyde resins starting with a precondensate, adding a salt and heating at 60.degree.-65.degree. C. while adding urea to bring the mole ratio formaldehyde to urea to 2.0:1 to 2.65:1. The pH is adjusted to 5 to 6.5 and the temperature raised to 80.degree. to 100.degree. C. and urea added to bring the ratio of formaldehyde to urea to from 1.5:1 to 2:1.